Color motion picture film is a relatively recent development. Before the advent of color film stock in the 1950s, a process for making color motion pictures included capturing color information on two or more reels of black and white film. In the original Technicolor three color film separation process, three reels of black and white film were loaded into a specially-designed movie camera. The light coming through the lens was split into the three primary colors of light and each was recorded on a separate reel of black and white film. After developing the three reels, three photographic negatives representing the yellow (inverted blue), the cyan (inverted red), and the magenta (inverted green) portion of the original reels were created.
In addition to the creation of color separations through the original Technicolor process, color separations also have been produced and used for the archival of color film because black and white film stock generally has a much greater shelf-life than color film. In this process, the color film stock is used to expose one reel of black and white film with sequential records of red, green, and blue so that each frame is printed three times on the resultant reel to form a sequential separation.
Film studios may recombine the three color separations onto a single reel of color film using a photographic process that is performed in a film laboratory. In the case of three color separations that are each located on a separate reel, an optical film printer is employed to resize and reposition each source reel, one at a time. In particular, three passes are made. First, the magenta source reel is projected through an appropriate color filter onto the destination reel. Thereafter, the destination reel is rewound, the next source reel is loaded and resized, and the color filter is changed. For this reel, a human operator determines a global alignment (and scaling if necessary) for the entire set of frames within the reel or, alternatively, within selected scenes on a scene-by-scene basis, with each scene including several, if not hundreds, of frames. However, because of the human intervention required, the alignment often is not determined on a frame-by-frame basis for the entire reel. The process is repeated until all three color separations have been printed on the single destination reel using the optical film printer. The resulting destination reel is called an interpositive (“IP”), and the colors are now represented as red, green, and blue (as opposed to cyan, magenta, and yellow).